Crystalline polypropylenes have been extensively used in the filed of various moldings, because of good mechanical properties, good chemical resistance or the like, very useful balance with economy. However, conventional known polypropylenes have limited use. Thus a strong desire has been directed to the improvement in performance of polypropylenes, especially the improvement in heat resistance characteristics such as high-temperature rigidity and high tenacity.
In recent years, it is known that propylene is polymerized using a catalyst consisiting of combinations of metallocenes with aluminoxanes to produce stereoregular polypropylenes with narrow molecular weight distributions. For instance, propylene is subjected to polymerization using a catalyst consisting of silylene-bridged metallocenes and aluminoxanes having the specific structures to produce high stereoregular polypropylenes with narrow molecular weight distributions, as disclosed in Japanese Patent Kokai 3-12406 and 3-12407 and CHEMISTRY LETTERS, PP. 1853-1856, 1989.
The polypropylenes produced by the above processes have narrow molecular weight distribution and high stereoregularity, and also higher melting point and higher rigidity than polypropylenes produced from the use of prior metallocene catalysts, but they have a double bond at one end of the polymer and may impair a chemical stability depending on the use condition, since the polymerization of propylene is performed in the absence of hydrogen. Thus the improvement of such processes has been desired with more improved heat resistance and higher tenacity.
Polypropylenes with narrow molecular weight distributions produced by known processes, due to the presence of a double bond at one end, have the problem that they are required to improve a chemical stability and a heat resistance and provide a high tenacity.